An irregular lamina has no proper geometrical shape and its thickness is negligible compared to its length and breadth. The volume of the irregular lamina is calculated using the formula

*Volume* = *Area* × *Thickness*

The thickness area of the lamina can be determined using screw gauge while the surface area of the lamina is measured using graph paper.

## Aim

To determine the volume of an irregular lamina using screw gauge.

## Apparatus/ Materials Required

- Screw Gauge
- Irregular lamina of uniform thickness
- A centimetre graph paper
- A pencil

## Theory

Screw gauge is an instrument that is used to accurately measure the volume of the irregular lamina. It is made of a U-shaped frame fitted to a screwed spindle that is attached to a thimble. A scale graduated in mm is engraved parallel to the axis of the thimble. This is known as the pitch scale. The head of the screw consists of a rachet that avoids undue tightening of the screw. On the thimble, there is a circular scale known as the head scale which is divided into 50 to 100 equal parts. The pitch of the screw is the distance moved by the spindle per revolution. The distance moved by the tip of the screw when the screw is turned through 1 division of the head scale is known as the least count.

**Formula:**

- \(Pitch\, of\, the\, Screw=\frac{Distance\,moved\,by\,the\,screw}{No\,of\,full\,rotations\,given}\)
- \(Least\,Count=\frac{Pitch}{Total\,Number\,of\,Divisions\,on\,the\,Circular\,Scale}\)

## Procedure

- Find the thickness of the lamina following the steps used to find the thickness of wire in the previous experiment
- Place the lamina on a clean cm graph and mark its boundary using a pencil.
- Determine the area of the lamina by counting the small squares enclosed on graph paper by the boundary of the lamina.

## Observation and Calculation

The readings of thickness at five different places along the edge of the lamina is noted down. Then the mean thickness is calculated. Make correction for zero error, if any.

From the outline of lamina drawn on the graph paper, note down the

Total number of complete squares = _____ cm^{2}

The volume of the lamina (area × thickness) = _____ cm^{3}

## Result

The volume of the given lamina is _____ cm^{3}.

## Viva Voce

1. Explain the reason why the screw used in the screw gauge is known as a micrometre screw.

**Answer:** It is known as a micrometre screw because it can measure distance correctly up to a micrometre.

2. What is the principle of screw gauge?

**Answer:** When a screw moves in a fixed nut, the translatory motion of the screw is proportional to the rotation given to the screw.

3. List the different types of motion possessed by a screw.

**Answer:** It possesses two motions, and they are:

(i) linear motion possessed by the axis

(ii) circular motion possessed by the surface

4. Define the pitch of the screw gauge.

**Answer:** The distance moved by the screw in one complete rotation is known as the pitch of the screw gauge.

5. What is the value of the least count in commonly used screw gauge?

**Answer:** The least count in commonly used screw gauge is 0.001 cm.

6. List the reasons for zero error.

**Answer:** The following are the reasons for zero error:

- Defect in the manufacturing
- Wear and tear of the screw

7. How can the backlash error be avoided?

**Answer:** It can be avoided by applying slight applying lateral pressure on the screw and by always turning in the same direction.

8. Which metal is used to make the screw and why?

**Answer:** The screw is made of gunmetal to avoid wear and tear after long use.

9. Define the least count of screw gauge.

**Answer:**The distance moved by the screw in one complete rotation is known as the least count.

10. What is ratchet? Explain its utility.

** Answer:**Ratchet is referred to as the arrangement inside the milled head at the end of the screw. It prevents the screw from undue pressure.

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